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Re: [PATCH] sysdeps/arm/armv7/multiarch/memcpy_impl.S: Improve performance.
- From: Will Newton <will dot newton at linaro dot org>
- To: "Joseph S. Myers" <joseph at codesourcery dot com>
- Cc: "libc-ports at sourceware dot org" <libc-ports at sourceware dot org>, Patch Tracking <patches at linaro dot org>
- Date: Fri, 30 Aug 2013 19:46:28 +0100
- Subject: Re: [PATCH] sysdeps/arm/armv7/multiarch/memcpy_impl.S: Improve performance.
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- References: <520894D5 dot 7060207 at linaro dot org> <Pine dot LNX dot 4 dot 64 dot 1308292353450 dot 1487 at digraph dot polyomino dot org dot uk> <CANu=DmgLOGuVi9gjdZ2iVHsPKbH2BWm=ykvJ3qxZ9pDL+H8oxg at mail dot gmail dot com> <Pine dot LNX dot 4 dot 64 dot 1308301513370 dot 17271 at digraph dot polyomino dot org dot uk>
On 30 August 2013 16:18, Joseph S. Myers <joseph@codesourcery.com> wrote:
> On Fri, 30 Aug 2013, Will Newton wrote:
>
>> > There are various comments regarding alignment, whether stating "LDRD/STRD
>> > support unaligned word accesses" or referring to the mutual alignment that
>> > applies for particular code. Does this patch make any of them out of
>> > date? (If code can now only be reached with common 64-bit alignment, but
>> > in fact requires only 32-bit alignment, the comment should probably state
>> > both those things explicitly.)
>>
>> I've reviewed the comments and they all look ok as far as I can tell.
>
> Are you sure? For example, where it says "SRC and DST have the same
> mutual 32-bit alignment", is it not in fact the case after the patch that
> they now have the same mutual 64-bit alignment, even if this code doesn't
> currently rely on that? I think the comments in each place should be
> explicit about both things - what preconditions the code relies on, and
> what possibly stronger preconditions are in fact true given the other code
> in this file. Saying the mutual alignment is 32-bit when the reader can
> see from the code not far above that it's 64-bit just seems liable to
> confuse the reader, even if the comment is still formally true.
> Similarly for the requirements on unaligned word accesses - after this
> patch, which uses of ldrd/strd do require that?
Yes, you're right, that needs more thought. I'll have a look at it next week.
--
Will Newton
Toolchain Working Group, Linaro